Inkjet printing apparatuses can be used for single or multicolor printing of a printing substrate, for example a single sheet or a belt-shaped recording material made of the most varied materials (paper, for example). The design of such inkjet printing apparatuses is known; see for example EP 0 788 882 B1. Inkjet printing apparatuses that operate according to the Drop-on-Demand (DoD) principle, for example, have as a printing unit a print head or multiple print heads with nozzles comprising ink channels, the activators of which nozzles—controlled by a printer controller—induce ink droplets in the direction of the printing substrate, which ink droplets are directed onto the printing substrate in order apply print dots there for a print image. The activators can generate ink droplets thermally (bubble jet) or piezoelectrically.
Given low print utilization of the inkjet printing apparatus, not all nozzles of the inkjet print heads are activated in the printing process; and many nozzles have idle time periods (print pauses), with the result that the ink in the ink channel of these nozzles is not moved. Due to the effect of the evaporation from the nozzle opening, the danger exists here that the viscosity of the ink then changes. This has the result that the ink in the ink channel can no longer move optimally, and for example cannot exit from the nozzle. In extreme cases, the ink in the ink channel dries completely and blocks the ink channel, such that a printing with this nozzle is no longer possible.
The drying of the ink in the nozzle can be prevented in that printing occurs from all nozzles within a predetermined cycle. This cycle can be adjusted corresponding to the print utilization. Individual points can thereby be applied in unprinted regions of the printing substrate, or print dot lines can be printed between print pages. In addition to unnecessary ink consumption and additional wear of the print heads, this method can lead to disruptions in the print image.
These problems in particular occur in color printers. Here, for example, print bars with print heads are arranged in a fixed position relative to one another as a printing unit. For example, print bars with five respective print heads can be provided, respectively one print bar for the colors black, cyan, magenta, yellow. Here the problem exists that one or more colors cannot be used, for example in black-and-white printing. Multiple cleaning cycles are then required in order to make the unused print heads run well again.
From U.S. Pat. No. 6,578,945 B2 it is known to avoid the drying out of the nozzles in an inkjet printing apparatus with multiple print heads in that the nozzles are closed with protective caps. The ink dispensed in the cleaning of the nozzles is thereby captured by the protective caps. In order to apply the protective caps onto the nozzles, the printing unit with the print heads is moved upward, away from the printing substrate; the protective caps are driven into the intervening space between printing unit and printing substrate; and thus the print heads are thereby cleaned. The protective caps are moved upward onto the print heads via an elastic force, wherein the print heads are covered. The protective cap unit remains in this position until the printing unit should be used again for printing.
In US 2007/0157962 A1, an inkjet printing apparatus is described in which the print head can be moved upward and perpendicular away from the printing substrate between a printing position and a position in which no printing is implemented. In this printing position a protective cap comprising rubber can be applied onto the nozzles of the print head.
From DE 10 2005 034 029 A1 an inkjet printing apparatus is known with multiple print heads arranged serially in the transport path of the printing substrate. The inkjet print heads successively print a resulting print image on the printing substrate. Individual inkjet print heads can be deactivated and shifted laterally (for example in order to service these) while the remaining print heads can continue printing.
DE 197 26 642 C1 describes a device to position an inkjet print head and a cleaning and sealing device. The inkjet print head can be pivoted from a printing position into a cleaning position and back again. A cleaning and sealing device can be moved onto the inkjet print head and away from this again. The cleaning and sealing device has a sealing cap and a wiping lip.